This has immensely buggued me too (the 4 char limit), because there are so many 3 char (or even 2 char) words/abbreviations we use regularly, and I can't find with a search (bots, OP, LTN, bob, rso, and many many others).
I had to resort a google search with site:forums.factorio.com in that case, but it lacks some useful features (show as topic, ...).

Loewchen wrote:The minimum character length per word is 4, and some common words are not indexed. Therefore "mod rso then between" will not find anything.

Shouldn't it show a "search term is too short" warning then instead of just "no results found"? That's how any other™ search function works.

And more on spot... is there any technical (non-performance) reason to not set that limit down to 2? The index would probably eat slightly more disk space, but that should be about it, shouldn't it?

Edit:
Did some googling and the results suggest that 4 is simply the default minimum value. So maybe it's time to change that ;)? At least 3 would be really good. Also supposedly as a hackaround one can append an asterisk to short search termns (LTN*, RSO*) to get at least some results, though with a worse signal/noise ratio.
And the optional non-indexing of common words is supposedly actually dynamic. So something like "bot" which is used here really often might not be indexed becaues it is used too often...

maybe one way could be to add a message in the results page, like "no results found, but click here to include 3 letter words", but it probably isnt as simple as that, since the whole data base will probably need reindexing (and back end code might not include those functions)